Ring laser gyros use a ring laser, usually imbedded in a dimensionally stable laser block having a low thermal expansion coefficient. The laser block typically has three or more substantially coplanar bores therein, each containing a laser gas such as helium and neon. At the junctions of the bores are corner mirrors that are spaced to define the laser cavity. Typically a cold cathode and a pair of anodes with a d.c. voltage between the cathode and the nodes are used to produce the lasing. The ring laser has two counterpropagating laser beams therein. As the laser is rotated about an axis, enclosed by the ring laser, the two beams separate in frequency. That difference in frequency is a measure of the angular velocity of the ring laser. For the ring laser to be used as a gyro, it is necessary to extract a portion of each of the two beams and to beat them against each other to obtain a heterodyne. The optical interference between the two beams produces fringes, and the fringes move with the angular velocity to be measured. A fringe counter may be used to determine the angular velocity or angle of displacement of the ring laser, and a ring laser gyro is thereby produced.
To extract a portion of the light of each beam, (on the order of 0.01%), one of the comer mirrors is partly transmissive. The transmitted light is then guided so that interference fringes may be produced. The prior art optics for extracting portions of the beams used a prism with total reflection of one of the two beams with two bounces for each beam before it strikes a beam splitter. The beam-splitter directs the light from both beams into a common optical sensor where the fringes per unit of time are counted. Frequently a second corner mirror is partly transmissive to extract a part of one or more of the beams. The light so-extracted can be used to control the inward and outward movement of a third corner mirror to control the cavity length of the ring laser.